With regard to the compound of this invention, there has already been established by the present inventors a method for the synthesis thereof in which is used the reaction of an ester of 2-haloethylmethylphosphinic acid with an ester of acetoaminomalonic acid (Japanese Provisional Patent Publication No. 48-91019/1973). Further, H. Gross et al. established a method by which the compound was obtained via .beta.-chloroethyl ester of .beta.-chloroethylmethylphosphinic acid [J. Pract. Chem., 318, 157 (1976)], and P. Mastalerz et al. carried out a synthesis of the present compound by using a dialkyl methylphosphonite and a dialkyl acetal of a 3-halopropanal [Rocz. Chem., 49, 2127 (1975)].